01 - Whisk together soy sauce, mirin, sake, sugar, sesame oil, minced garlic, grated ginger, sesame seeds, and sliced green onion in a mixing bowl until sugar dissolves completely.
02 - Add sliced beef to the marinade, tossing thoroughly to coat each piece. Let stand for at least 10 minutes at room temperature, or refrigerate for up to 1 hour for deeper flavor penetration.
03 - Preheat a grill pan or tabletop grill over high heat until smoking hot. The surface should be sufficiently hot to achieve quick searing and caramelization.
04 - Place marinated beef slices on the hot grill. Cook for 1 to 2 minutes per side until just cooked through and developing a caramelized exterior. Avoid overcooking to maintain tenderness.
05 - Arrange sliced onion, bell pepper, shiitake mushrooms, and zucchini on the grill alongside the beef. Cook until tender-crisp with appealing char marks, approximately 3 to 4 minutes.
06 - Transfer grilled beef and vegetables to a serving platter. Serve immediately alongside steamed white rice while hot.